⬆️ Drop support for Python 3.7, require Python 3.8 or above (#10442)
* 📝 Update docs, require Python 3.8+, drop 3.7
* 🔧 Update pyproject.toml, drop support for Python 3.7, require Python 3.8+
* 👷 Update CI GitHub Actions, drop support for Python 3.7, require 3.8+
* 📝 Update docs' references to Python 3.6 and 3.7, use Python 3.8
